Wind turbines are designed to operate reliably and safely under a wide range of wind conditions. However, despite careful design practices some wind conditions may result in oscillations of the wind turbine components, particularly the blades. Such oscillations can compromise the reliable operation of the wind turbine, and this is particularly the case if the oscillations coincide with resonant frequencies of the wind turbine. Since resonant oscillations can lead to damage of wind turbine components, much effort is made to measure potentially dangerous vibrations of components and to mitigate their effect.
A source of vibration that is not currently particularly well understood, measured, or managed is vibration of wind turbine blades in the edgewise direction. Edgewise vibration at certain frequencies can cause damage to the blades but also to other components of the wind turbine.
Some attempts to quantify edgewise vibration have been made using sensors incorporated into the blades. However, a large number of sensors are necessary that are often expensive and that have to be incorporated into the blade carefully so as not to be in the way of other features such as lightning protection systems. Additionally, for older wind turbines, retrofitting sensors is much more expensive and can compromise the structure of the blade.
It is the aim of the present invention to address at least some of the abovementioned issues.